Radar system

ABSTRACT

A scanning radar system suitable for detecting and monitoring ground-based targets includes a frequency generator, a frequency scanning antenna, and a receiver arranged to process signals received from a target so as to identify a Doppler frequency associated with the target. The frequency generator generates sets of signals, each set having a different characteristic frequency, and includes a digital synthesiser arranged to modulate a continuous wave signal of a given characteristic frequency by a sequence of modulation patterns to generate one set of signals. The frequency scanning antenna cooperates with the frequency generator to transceive radiation over a region having an angular extent dependent on the generated frequencies. Digital synthesiser techniques capable of precise frequency generation and control are combined with passive frequency scanning and Doppler processing techniques, enabling accurate control of range and scan rates, and optimisation of range cell size for factors such as slow and fast target detection and Signal to Noise ratio, so that targets can be detected at distances considerably farther away than is possible with known systems having similar power requirements.

BACKGROUND OF THE INVENTION

The present invention relates to a radar system, and relatesspecifically to scanning radar systems that are particularly, but notexclusively, suitable for detecting and monitoring ground-based targets.

Radar systems are used to detect the presence of objects and to measurethe location and movement of objects. In general, radar systems aredesigned for a specific application: to measure distance over aspecified range of distances; over a specified scan region; within aspecified level of accuracy; and in relation to a specified orientation.For radar systems that are required to scan over large distances, theantennas are required to generate powerful electromagnetic radiation,requiring the use of a correspondingly powerful source and specifictypes of antennas.

It is common for such radar systems to sweep across a given region,scanning the region for the presence of such objects. In order to sweepover the region the radar systems either employ mechanical devicescomprising an antenna that physically moves in space, or electronicdevices comprising elements that are arranged to steer radiation as itis transmitted or received. A problem with the mechanical radar systemsis that their operation is reliant on physical components and associatedcontrol and moving parts. This inventory of parts is costly and canrequire a commensurately large power source.

One known group of electronic devices is phased antenna arrays, whichapply various phase shifts to signals, thereby effectively steering thereceived and transmitted beams. These electronic devices are commonlyused in RF sensor and communications systems because they do not involvephysical motion of the antenna and are capable of moving a beam rapidlyfrom one position to the next. Whilst radar systems incorporating suchdevices can provide an extremely accurate measure of the position oftargets, a problem with these types of electronic devices is thatadequate control of the beam requires often several arrays ofelectronics components; this increases the physical size, complexity andcost of the radar system.

Another group of such electronic devices is frequency scanning arrays,which, in response to input signals of varying frequencies, can steer abeam in an angular plane. Frequency scanning arrays have been combinedwith moving parts that rotate in another plane, as described in U.S.Pat. No. 4,868,574. However, a problem with this combination is that itincurs the size and cost shortcomings of regular mechanical scanningsystem and performance-wise, is less accurate than the phased antennasystems.

It will therefore be appreciated that the various known radar systemsare one or several of costly, bulky and heavy, which limits theirapplicability to uses in which either cost or weight or size are anissue.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a scanning radar system comprising a frequency generator, afrequency scanning antenna, and a receiver arranged to process signalsreceived from a target so as to identify a Doppler frequency associatedwith the target. The frequency generator is arranged to generate aplurality of sets of signals, each set having a different characteristicfrequency. The frequency generator includes a digital synthesiserarranged to modulate a continuous wave signal of a given characteristicfrequency by a sequence of modulation patterns whereby to generate theset of signals. The frequency scanning antenna is arranged to cooperatewith the frequency generator so as to transceive radiation over a regionhaving an angular extent dependent on the generated frequencies.

The inventors of the present invention have focussed the design efforton low power radar systems that are capable of detecting and locatingobjects moving along the ground, and to this end have combined digitalsynthesiser techniques, which are capable of precise frequencygeneration and control, with passive frequency scanning and Dopplerprocessing techniques. This enables accurate control of range and ofscan rates, and enables optimisation of range cell size for factors suchas slow and fast target detection and Signal to Noise ratio, and thusenables detection of targets located at distances considerably fartheraway than is possible with known systems having similar powerrequirements.

With scanning radar systems there is inherently a trade off between therate at which an area is scanned and the range, or distance, over whichtargets can be detected during the scan. For relatively fast scan rates,a given angular region can be scanned several times as a target movesrelative to the region, but the number of signals in a given set ofsignals will be correspondingly limited, with the result that thedetectable range will be limited. For relatively slow scan rates, thenumber of signals in a given set of signals is relatively high, meaningthat targets located further away can be detected, at the expense oftracking movement of targets within the angular region. Advantageouslythese parameters can be accurately and repeatably controlled by thedigital synthesiser, while use of a frequency scanning radar means thatthe radar system can return to transmit at precisely the same angle atwhich signals have previously been transmitted, thereby reducing errorsin range return associated with Doppler modulation that are associatedwith mechanical scanning radar systems.

The range R_(max) of the scanning radar system according to embodimentsof the invention can be estimated from the radar equation R_(max)=(P_(t)G A_(e)σ/((4π)² S_(min)))^(1/4), where S_(min) is the minimum detectablesignal (as a power value), P_(t) is the transmitted power, G is the gainof the antenna, A_(e) is the effective aperture of the antenna and σ isthe cross sectional area of the target. For a target having a crosssectional area of approximately 1 m², the maximum range R_(max) isapproximately 5 km; for targets such as cars, which present a crosssectional area of approx 10 m², the maximum range R_(max) isapproximately 9 km while for larger targets having a cross sectionalarea of the order 100 m², the maximum range R_(max) is approximately 15km. It will be appreciated that as the scanning duration in a particulardirection increases, the value of S_(min) will decrease; accordingly,for a given target size, the radar range in respect of that target inthe particular direction will increase until, due to systemimperfections, target motion and Doppler spread, further scanning inthat direction cannot reduce S_(min) or increase Pt any further to havea material bearing on R_(max).

Since signal strength is proportional to distance to the fourth powerfrom source, an advantage of designing a short range radar system isthat the power required to transmit radiation within a range of tens ofkm requires less power than conventional radars typically require.Consequently the weight and required output of the power sourcecomponents is less than that required by conventional radar systems.

The radar system might be physically located on the ground or sited uponan object that is itself grounded (such as on a floor of a building orupon a vehicle).

A further advantage of embodiments of the invention is that frequencyscanning antennas are less complex, in terms of processing and controlcomponents, than phased antenna arrays or mechanical steering antennas.As a result, the size and weight of the antenna circuit components arerelatively small and light, respectively. These factors together enablethe radar system to be powered by for example a 12 Volt battery, a solarpanel or a vehicle battery (e.g. via a convenient connection within thevehicle such as a cigarette lighter) such as a 12, 24 or 48 Volt vehiclebattery.

In one arrangement the radar system is arranged to transmit dataindicative of radiation received and processed thereby to a remoteprocessing system for display, review and interpretation at the remoteprocessing system instead of at the radar system, thereby furtherreducing the processing and control components required by the radarsystem. Advantageously, and as will be appreciated from the foregoing,since a radar system according to this aspect of this invention isneither bulky nor heavy, it readily lends itself to portability.

Having selected a frequency scanning antenna, the inventors were facedwith the problem of identifying a frequency source that minimises theamount of phase noise in the signal, so as to enable discriminationbetween small targets that move and large stationary targets. Most knownsynthesisers utilise a fixed frequency source (e.g. in the form of acrystal oscillator), which, in order to generate a range of frequencies,are integrated with a circuit that includes frequency dividers and avariable frequency oscillator (conventionally referred to as PhaseLocked Loop Synthesisers). Such variable frequency oscillatorsinherently have a certain amount of phase noise (typically referred toas dither) in the output signals, and phase locked loop synthesisersmultiply up the signal received from the signal generator, including thenoise. As a result, a signal with a significant amount of dither, whenreflected from a stationary target, can confuse the signal processingcomponents and appear as a moving target.

Preferably, therefore, the frequency generator is embodied as a signalgenerator comprising a first circuit portion and a second circuitportion, wherein the first circuit portion includes a variable frequencyoscillator arranged to output signals at an output frequency independence on control signals input thereto and tuning means arranged togenerate the control signals on the basis of signals received from thesecond circuit portion for use in modifying operation of the variablefrequency oscillator, and the second circuit portion is arranged toreceive the output signals and to derive therefrom signals to be inputto the tuning means, the second circuit portion comprising a frequencydivider arranged to generate signals of a divided frequency, lower thanthe output frequency. The second circuit portion also includes meansarranged to derive reduced frequency signals from the output signal, thereduced frequency signals being of a frequency which is lower than theoutput frequency and higher than the divided frequency.

Conveniently the frequency generator further includes a fixed frequencyoscillator such as a crystal oscillator or SAW (Surface Acoustic Wave)oscillator, which provides input to the first circuit portion. In onearrangement the tuning means of the first circuit portion preferablycomprises a further frequency divider and a phase comparator, and thesecond circuit portion comprises a static frequency multiplier and amixer which cooperate so as to reduce the frequency of signals that areinput to the frequency divider of the second circuit portion. Thefurther frequency divider associated with the first circuit portion isemployed to step-down the fixed oscillator frequency, so as to controlthe frequency resolution of the signal generator. In relation to thesecond circuit portion, the frequency divider is employed to step-downthe frequency of signals output from the mixer, and the output from thefirst and second frequency dividers are synchronised by a phasecomparator, which generates the control signals (in the form ofphase-error signals) to modify the output of the voltage controlledoscillator. Conveniently the second circuit portion serves to reduce thefrequency of signals that are input to the second frequency divider,which means that the amount of multiplication required by the secondfrequency divider is correspondingly reduced.

Since phase noise is dependent on the amount by which the frequency of agiven signal is multiplied, substantially less phase noise is present inthe signals generated by embodiments of the invention compared with thatgenerated by conventional signal generators. In addition, this meansthat phase locked loops of signal generators embodied according to theinvention are capable of operating at higher loop frequencies than ispossible with conventional arrangements.

Essentially, therefore, the inventors identified a specific arrangementof components which minimises the amount by which output from thefrequency source is multiplied, and thus the amount of phase noise thatis transmitted.

In preferred arrangements, the scanning radar system is a FrequencyModulated Continuous Wave (FMCW) radar system, which is arranged tooutput a frequency modulated signal of a predetermined pattern,preferably comprising a sequence of linear frequency sweeps. In a mostconvenient arrangement the digital synthesiser is responsive to inputsso as to repeat the modulation pattern a predetermined number of times.

Radar systems are commonly used to identify the Doppler frequency oftargets so as to identify the magnitude and direction of movementthereof. The inventors have identified a problem with FMCW radarresulting from the fact that a target's Doppler frequency is dependenton the radar's carrier frequency, namely that a radar which operateswithin a range of frequencies can generate Doppler frequencies for agiven target which can, of themselves, indicate movement of the target.The inventors realised that by varying the period of the frequencysweeps in proportion to the carrier frequency, the normalised Dopplerfrequency remains substantially constant.

Accordingly in relation to this aspect of the present invention, theinventors have developed a frequency scanning radar controller for usein controlling frequency modulation of a continuous wave signal, thecontinuous wave signal having a characteristic frequency and beingmodulated by a sequence of modulation patterns, wherein the radarcontroller is arranged to modify a given modulation pattern independence on the characteristic frequency of the signal beingmodulated.

In preferred embodiments of the invention the radar controller isarranged to modify the duration of individual patterns in the sequence,thereby modifying the modulation pattern. In one arrangement eachmodulation pattern of the sequence comprises a linear ramp period and adwell period, and the radar controller is arranged to modify theduration of dwell periods of respective modulation patterns in thesequence, thereby modifying the modulation pattern. In anotherarrangement each modulation pattern of the sequence comprises a linearramp period and a descent period, and the radar controller is arrangedto modify the duration of descent periods of respective modulationpatterns in the sequence, thereby modifying the modulation pattern. Inother arrangements the modulation pattern includes a combination of aramp period, a descent period and a dwell period, in which case theduration of either of the descent or dwell periods can be modified.

Conveniently the frequency generator is responsive to inputs indicativeof the respective durations so as to modulate the characteristicfrequency.

In relation to the frequency scanning antenna, the inventors found thata particularly efficient antenna (in terms of level ofcomplexity—relatively low—and performance—relatively good) is thetravelling wave antenna. The inventors were then faced with the problemthat a travelling wave antenna only radiates over a relatively narrowscan angle as the frequency is changed, this limiting the scan area overwhich the antenna could be used.

The inventors realised that two or more array antennas could be arrangedto form an antenna structure, and that, by coordinating the feed to arespective antenna array of the antenna structure, individual scan areascan be combined to generate an increased overall scan region.

Accordingly the inventors developed a frequency scanning antennastructure for transceiving radio frequency energy and being capable ofsteering a radio frequency beam to a plurality of different angles aboutthe antenna structure, the antenna structure comprising at least twoarray antennas and a controller for controlling input of energy to thetwo array antennas, wherein the array antennas are disposed within theantenna structure such that the antenna structure is capable of steeringa beam to a first angle using one of the two array antennas and ofsteering a beam to a second angle, different to the first angle, usingthe other of the two array antennas.

In one arrangement the antenna structure is arranged to steer a beamacross a plurality of non-contiguous angular regions, and in another tosteer a beam across a contiguous angular region. Conveniently theantenna structure is capable of steering a beam across a first range ofangles (a first angular region) using one of the two array antennas andof steering a beam across a second range of angles (second angularregion) using the other of the two array antennas: the first and secondangular regions being different, and collectively offering a scan regionof an angular extent greater than that achievable with individualantenna arrays.

Conveniently each array antenna comprises input means for inputting theenergy thereto, and the controller is arranged to input energy torespective array antennas so as to steer the beam to the first andsecond angles. More specifically, each input means is arranged to inputenergy to respective array antennas so as to steer the beam across thecontiguous or non-contiguous angular regions. In one arrangement theinput means is connectable to ends of the antenna array and is inoperative association with a frequency generator—such as that describedabove—so as to receive signals comprising radio frequency energy at aplurality of different frequencies in order to steer the beam.

Preferably the controller is arranged to input energy in accordance witha predetermined sequence so as to steer the beam across the first andsecond angles, the sequence comprising, for example, inputting energy toa first end of the first antenna array, inputting energy to a first endof the second antenna array, inputting energy to a second end of thesecond antenna array, and inputting energy to a second end of the secondantenna array.

In relation to the configuration of the antenna structure itself, theantenna structure can conveniently be characterised in terms of alongitudinal axis and a transverse axis perpendicular to thelongitudinal axis: a first of the array antennas being inclined at thefirst angle relative to the transverse axis and a second of the arrayantennas being inclined at the second angle relative to the transverseaxis. Moreover, the first and second array antennas are symmetricallydisposed about the longitudinal axis and each of the array antennascomprises two ends and two side portions, a side portion of the secondarray antenna substantially abutting a side portion of the first arrayantenna. The extent of the scan region is dependent on the physicalrelationship between the two array antennas, more specifically on theangle each respective array antenna makes to the transverse axis. In onearrangement the angular extent of the radar system is substantially 80degrees, but other angles are possible, ranging from 60 degrees, 100degrees, 120 degrees, consistent with various arrangements of theantenna arrays within the antenna structure. Furthermore the antennastructure can be configured so as to include more than two arrayantennas, thereby further increasing the angular extent of the radarsystem.

In one arrangement, each of the array antennas comprises a meshstructure and a dielectric base. Each mesh structure can comprise aplurality of interconnected elements embodied as a micro circuit strip(commonly called a microstrip) and can conveniently be disposed on asurface of a corresponding dielectric base.

The mesh structure can conveniently be characterised by the lengths ofrespective sides and ends of the elements: each of the elementscomprising two sides and two ends of respective lengths, the length ofthe sides being greater than the length of the ends. Typically thelength of the sides is of the order of one wavelength at a mid-pointbetween the first frequency and the second frequency and the length ofthe ends is of the order of one-half of one wavelength at the mid-pointfrequency. Each mesh element has a characteristic width, and in apreferred arrangement the mesh widths of the sides are progressivelydecreased from the centre of the mesh to each respective end thereof.Since impedance is inversely proportional to mesh width, it will beappreciated that this provides a convenient means of controlling theimpedance of the antenna array elements and thus the resulting radiationpattern.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing components of a radar systemaccording to embodiments of the invention;

FIG. 2 is a schematic block diagram showing an arrangement of componentsof a frequency generator shown in FIG. 1;

FIG. 3 is a schematic diagram showing a modulation pattern for use bythe frequency generator of FIG. 2;

FIG. 4 is a schematic block diagram showing an alternative arrangementof components of a frequency generator shown in FIG. 1;

FIG. 5 a is a schematic diagram showing an embodiment of an antennaarray utilised in the antenna shown in FIG. 1;

FIG. 5 b is a schematic diagram showing another embodiment of an antennaarray utilised in the antenna shown in FIGS. 1 and 4;

FIG. 6 is a schematic diagram showing components of a radar systemaccording to an alternative embodiment of the invention;

FIG. 7 is a schematic engineering drawing showing an antenna structurecomprising the antenna arrays of FIG. 5 a or 5 b for use in either ofthe radar systems shown in FIG. 1 or 6;

FIG. 8 a is a schematic diagram showing radiation emitted from theantenna structure of FIG. 7 for a given output frequency;

FIG. 8 b is a schematic block diagram showing radiation emitted from theantenna structure of FIG. 7 for two different output frequencies;

FIG. 9 is a schematic block diagram showing components of a radar systemaccording to yet another embodiment of the invention;

FIG. 10 is a schematic engineering drawing showing an antenna structurecomprising the antenna arrays of FIG. 5 a or 5 b for use in any of theradar systems shown in FIGS. 1, 6 or 9;

FIG. 11 is a schematic diagram showing radiation emitted from theantenna structure of FIG. 10;

FIG. 12 is a schematic flow diagram showing steps performed by thecontroller shown in FIG. 1 during scanning of the radar system of FIG.1;

FIG. 13 is a schematic diagram showing processing of signals in relationto a transmitted modulation pattern;

FIG. 14 is a schematic block diagram showing components of a radarsystem according to yet another embodiment of the invention;

FIG. 15 is a schematic block diagram showing components of a radarsystem according to a yet different embodiment of the invention; and

FIG. 16 is a schematic engineering drawing showing an alternativeantenna structure comprising the antenna arrays of FIG. 5 a or 5 b foruse in either of the radar systems shown in FIG. 1 or 6.

Several parts and components of the invention appear in more than oneFigure; for the sake of clarity the same reference numeral will be usedto refer to the same part and component in all of the Figures. Inaddition, certain parts are referenced by means of a number and one ormore suffixes, indicating that the part comprises a sequence of elements(each suffix indicating an individual element in the sequence). Forclarity, when there is a reference to the sequence per se the suffix isomitted, but when there is a reference to individual elements within thesequence the suffix is included.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a radar system 1 according to embodiments of the invention,comprising a power source 10, a controller 12, and a computer 14, thepower source and computer 10, 14 being arranged to provide power to, andoperational control over, the controller 12. The controller 12 comprisesa microprocessor and a set of instructions (not shown) for executionthereby, effectively generating control signals that cause the RFfrequency source, or signal generator 16, to output RF energy at aspecified frequency F_(OUT), and this output signal, under control ofswitches 18 and amplifiers 20, drives antenna 22 (whilst the Figureshows a switch component 18, it will be appreciated that in thisparticular arrangement—in which there is only one antenna 22—the switch18 is inessential). As will be described in more detail below, the RFfrequency source 16 generates signals within a range of frequencies,causing the antenna 22 to transmit beams in different angulardirections, thereby scanning over a region beyond the radar system 1.

The radar system 1 also includes a receiving antenna 32, which receivesradiated signals reflected back from objects, and passes the receivedradiation through switch and amplifier components 18′, 20′ to mixer 34.The mixer 34 comprises two inputs: a first connected to the RF source16; and a second connected to the receiving antenna 32. The output ofthe mixer 34 is fed to an Analogue to Digital converter ADC 36, toproduce a digitised signal for input to the signal processor 38, whichperforms analysis of the received signal. The signal processor 38performs a spectral analysis on the received signals, because the rangebetween the radar system and external (reflecting) objects is containedas frequency information in the signal. Aspects of the receiving andprocessing components are described in detail below, but first aspectsof the RF frequency source and antenna will be described.

FIG. 2 shows components of the RF frequency generator 16 according to anembodiment of the invention, which is preferably used to generatesignals having a range of frequencies. Referring to FIG. 2, thefrequency generator 16 comprises a frequency source 200, first circuitportion 210 and a second circuit portion 220. The first circuit portion210 comprises a frequency divider 205, a phase comparator 209, a filter211, and a Voltage Controlled Oscillator VCO 213, while the secondcircuit portion 220 comprises a frequency divider 207, static multiplier201 and a mixer 203. The mixer 203 receives, as input, signals outputfrom the VCO 213 and signals from the high grade, static multiplier 201,and generates signals of frequency equal to the difference between thefrequencies of the two inputs (f₃). The values R1, R2 characterising thefrequency dividers 205, 207 are selectable, and the phase comparator 209is arranged to compare the frequency and phase of signals output fromthe frequency dividers 205, 207 (f₃/R2 and f_(ref)), so as to output aphase-error signal, of magnitude dependent on the difference betweenf₃/R2 and f_(ref). The phase-error signal is input to the VCO 213, andthe first circuit portion 210 operates so as to cause the output fromthe VCO 213 to stabilise in dependence on the phase-error signal. Thusdifferent values of R2 can be used to force the loop to stabilise at afrequency multiple of the input signal. In one arrangement the frequencysource 200 is embodied as a crystal oscillator and in anotherarrangement as a SAW oscillator.

As stated above, an objective of the design of the RF frequencygenerator 16 is to minimise the amount of phase noise present in theoutput signal F_(OUT). As will be appreciated from earlier parts of thisspecification, multiplication of the phase noise of the referenceoscillator and phase comparator is dependent on the magnitude of R1 andR2, so an objective of the RF frequency generator 16 is to minimise theamount of multiplication of the oscillator output 200—in other words tokeep the values of R1 and R2 as low as possible.

Accordingly the frequency generator 16 includes high-quality multiplier201 and mixer 203, the former (201) being arranged to increase thefrequency of the signal output from oscillator 200 to as high a value aspossible (e.g. the lower limit of the desired output frequency of VCO213), while the mixer 203 serves to output signals of frequency equal tothe difference between f₂ and f₁, thereby effectively stepping down theoutput of the VCO 213. As a result, the magnitude of the frequency inputto divider 207 is relatively low, which means that for tuning of theoutput of VCO 213, the value of R2 can be far lower than that possiblewith conventional arrangements.

The advantages of embodiments of the invention can best be seen withreference to a particular example, considering firstly how signals areprocessed by conventional phase-locked loop circuits and then howsignals are processed by embodiments of the invention, assumingfrequency source 200 outputs signals with a frequency of 100 MHz:

VCO 213 Output R1 R2 Conventional   5 GHz 1 50  5.1 GHz 1 51 5.11 GHz 10511 Embodiments of   5 GHz 1 10 the Invention  5.1 GHz 1 11 n = 40 5.11GHz 10 111 Embodiments of  5.1 GHz 1 1 the Invention 5.11 GHz 10 10 n =50

It can be seen that by stepping up the frequency of signals input fromthe frequency source 200 and mixing them with the output of the VCO 213,the amount of multiplication applied by the frequency dividers 205, 207,and thus amplification of phase noise in the oscillator output, iscorrespondingly reduced compared to conventional frequency synthesisers.It is to be noted that the circuit design shown in FIG. 2 offers a 20-30dB reduction in noise contribution of the phase detector compared toconventional circuits operating loop frequencies of the order 200 MHz.

The signals output from the second circuit portion are then modulated byoutput f_(DDS) of a third circuit portion 230, which in one arrangementcomprises a Direct Digital Synthesiser 223, a Digital to AnalogueConverter DAC 225 and a low pass filter 227. The third circuit portion230 is configured, under control of the controller 12 shown in FIG. 1,to generate a repeating pattern comprising a linear frequency ramp. Theramp has a specified duration and magnitude, values of which areprogrammed via the controller 12. FIG. 3 shows an example of one suchfrequency ramp 301 ₁ for a given carrier frequency f_(c1), the durationof which is approximately 64 μs, the magnitude of which, in terms ofrange of frequencies (f_(DDS,max)−f_(DDS,min)), is approximately 20 MHz,and is followed by a flyback ramp 303, to prepare the third circuitportion 230 for the next ramp 301 ₂. The pattern repeats at apredetermined rate—in the present example a rate of 8 KHz (thus a sweeprepeat period 307 of 125 μs (subject to the modifications describedlater in the specification)) is a convenient choice. Such a modulationpattern is entirely conventional and the foregoing details are includedas illustrative; the skilled person will appreciate that any suitablevalues could be selected, dependent upon the use of the radar system(e.g. the nature of the targets to be detected). For each carrierfrequency, the third circuit portion 230 is arranged to repeat thelinear ramp pattern a specified number of times, e.g. 256 or 512 times,the number being dependent on the desired signal to noise ratio andtherefore a design choice. Whilst the third circuit portion 230 shown inFIG. 2 comprises digital synthesiser components, it could alternativelybe embodied using analogue components such as a sawtooth generator andVCO or similar. Preferably, and in order to save power, it is to benoted that the antenna 22 is not energised during either of the flybackramp or dwell periods 303, 305.

Turning back to FIG. 2, the output f_(DDS) of the third circuit portion230 is input to a fourth circuit portion 240, which comprises a phasecomparator 233, a filter 235, a Voltage Controlled Oscillator 237 and amixer 231. The mixer receives signals output from the second circuit(having frequency f₂) and signals output from the VCO 237 (havingfrequency f₅) and outputs a signal at a frequency equal to thedifference in frequency between f₂ and f₅. The phase comparator 233outputs a phase-error signal, of magnitude dependent on the differencebetween (f₂−f₅) and f_(DDS) to the VCO 237, and the fourth circuitportion 240 operates so as to cause the output from the VCO 237 tostabilise accordingly.

The signals output from the fourth circuit portion 240 (having frequencyf₅) are then combined, by means of mixer 241, with signals of areference frequency f₄, which are signals output from the oscillator 200having been multiplied by a second static multiplier 251, and the outputis filtered (filter 243) so as to generate a signal having an outputfrequency F_(OUT). It will be appreciated from FIG. 2 that when thesignal generator 16 is operable to output signals corresponding to acarrier frequency of between 15.5 GHz and 17.5 GHz, for a crystaloscillator 200 outputting signals of frequency 100 MHz, the secondstatic multiplier 251 is of the order 130.

Whilst the signal generator 16 could be used to generate frequencieswithin any selected range of frequencies, when used as a ground-basedradar system, the frequency range can fall within the X band (8 GHz-12.4GHz); the Ku band (12.4 GHz-18 GHz); the K band (18 GHz-26.5 GHz); orthe Ka band (26.5 GHz-40 GHz), and most preferably within the Ku band,or a portion within one of the afore-mentioned bands. Thus for eachcarrier frequency the frequency generator 16 generates a repeatingpattern of frequency modulated signals of various carrier frequencies.

Whilst in preferred arrangements the first and second circuit portions210, 220 of frequency generator 16 are embodied as shown in FIG. 2, thefrequency generator 16 could alternatively be based on an arrangementcomprising a plurality of fixed frequency oscillators, as shown in FIG.4, one of which is selected via switch 400 so as to generate a signal atfrequency f₂. Judicial selection of an appropriate fixed frequencyoscillator (e.g. a crystal oscillator) means that the frequencygenerator 16 can incur minimal phase noise, since the signals are takendirectly from one of the oscillators. However, this advantage isaccompanied by a corresponding limitation, namely that there is no meansfor fine-tune adjustment of the carrier frequency, which can be adisadvantage when working with antennas 22 that require fine tuning ofthe carrier frequency to achieve optimal beamwidth distribution (interms of distribution of radiation within the lobes).

It will be appreciated from the foregoing that the antennas 22, 32transmit and receive radiation in response to input signals of varyingfrequencies; accordingly the antennas 22, 32 are of the frequencyscanning antenna type. In a preferred embodiment, the frequency scanningantenna is embodied as a travelling wave antenna structure comprising atleast two array antennas, one such antenna array 500 being shown in FIG.5 a. In one arrangement, the antenna array comprises a mesh structure501 and a dielectric base 503 and has input means 507 for inputtingenergy to the mesh structure 501. Preferably the antenna array 500 alsoincludes a ground plane. The input means 507 can comprise coaxial feedspositioned orthogonal to the plane of the antenna array 500, but theskilled person will appreciate that alternative feeds could be used.

In the arrangement shown in FIG. 5 a, each mesh structure 501 comprisesa plurality of rectangular interconnected elements 509 that are disposedon a surface of the dielectric base 503; each rectangular element 509comprises two sides 513 a, 513 b and two ends 511 a, 511 b, the length Lof the sides 513 a, 513 b being greater than the length S of the ends511 a, 511 b. The physics underlying the operation of the travellingwave antenna are well known, having first been investigated by JohnKraus and described in U.S. Pat. No. 3,290,688. Suffice to say that thelength L of the sides 513 is of the order of one wavelength of the meancarrier frequencies, and the length S of the ends 511 is of the orderone half of the wavelength of the mean carrier frequencies. It will beappreciated from the teaching in U.S. Pat. No. 3,290,688 that meshconfigurations other than rectangular and planar can be used.

In relation to the particular configuration adopted for embodiments ofthe invention, when current is fed through the mesh structure 501 viafeed 507, currents passing through the ends 511 a, 511 b are in phasewith one another. The current flowing through a respective side 513 a ofa given element 509 is received from an end 511 a of an adjacent element(shown as input 517) and splits into two current flows, each flowing ina different direction and being out of phase with one another. As isalso shown in FIG. 5 a, the width of the mesh making up sides 213 a, 213b is progressively decreased from the centre of the mesh to eachrespective end thereof, thereby effectively increasing the length of thesides 213 a, 213 b from the centre of the array towards its ends. In apreferred arrangement the antenna can be embodied as a micro circuitstrip.

The configuration of the antenna structure 701 according to anembodiment of the invention will now be described with reference toFIGS. 6 and 7. FIG. 6 shows a development of the radar system 1 shown inFIG. 1, including two antennas 22 a, 22 b rather than one. Turning alsoto FIG. 7, each of the antennas 22 a, 22 b is embodied in the form ofantenna array 500 a, 500 b shown in FIGS. 5 a and 5 b, and the antennastructure 701 is responsive to input from the controller 12 forcontrolling input of energy to respective feeds I₁, I₂ of the antennaarrays 500 a, 500 b. Referring also to FIG. 8 a, the two planar arrayantennas 500 a, 500 b are disposed within the structure 701 such that,for any given radio frequency, the antenna structure 701 is capable oftransmitting the radio frequency energy within different angular regions801 a, 801 b.

Referring back to FIG. 7, the antenna structure 701 can be characterisedby a longitudinal axis A1 and a transverse axis A2, which provides aconvenient frame of reference for describing the arrangement of theplanar antenna arrays 500 a, 500 b. As can be seen from FIG. 7, thefirst array antenna 500 a is inclined at an angle α relative to thetransverse axis A2 and the second planar array antenna 500 b is inclinedat angle β relative to the transverse axis A2. As can also be seen fromthe Figure, a side portion of the second array antenna 500 b abuts aside portion of the first array antenna 500 a (in the Figure the sideportions are located on the dot indicating axis A1) such that whenviewed face on, the antenna arrays 500 b are located in adjacentlongitudinal planes.

It will be appreciated from the schematic shown in FIG. 8 a that theorientation of the respective antenna arrays 500 a, 500 b—that is to sayangles α and β—determine the direction in which radiation is emittedfrom the antenna structure 701. Thus, by varying the relative positionsof the respective antenna arrays 500 a, 500 b, different portions of anangular region can be scanned for a given output frequency, f_(OUT,1).

FIG. 8 b shows radiation emitted 801 a-801 d from the antenna arrays fortwo different output frequencies f_(OUT,1), and f_(OUT,2), and it can beseen that appropriate selection of the values of f_(OUT,1) andf_(OUT,2), results in the antenna structure 701 outputting radiation soas to cover a substantially contiguous region, thereby scanning over agreater angular region than is possible with a single antenna array, oreven two arrays that are positioned in the same plane, such as thatdescribed in U.S. Pat. No. 4,376,938.

The arrangements shown in FIGS. 5 a, 6, 7, 8 a and 8 b relate to anarrangement in which the antenna arrays 500 a, 500 b comprise a singlefeed I₁, I₂ at one end of respective antenna arrays. However, andreferring to FIGS. 5 b and 10, each antenna array could comprise anadditional feed at its other end (I_(1,2), I_(2,2)). Each antenna 22 a,22 b can then be considered to be capable of emitting radiation in twodirections for a given frequency f_(OUT), since the transceive-behaviourof the antenna array 500 a is dependent on the direction from whichenergy is fed into the antenna. In FIG. 9, this is indicated by thepresence of two antennas for each of antenna parts 22 a and 22 b.Turning to FIG. 11, it can be seen that by feeding energy to two inputfeed points for each antenna array, the region R within which radiationcan be transceived is effectively doubled.

It will be appreciated from the foregoing that the frequency f_(OUT) ofsignals output from the signal generator 16 is controlled by thecontroller 12. In addition to controlling the duration and rate of theramp as described above, the controller 12 is arranged to select adifferent value for carrier frequency after the ramp pattern has beenrepeated a specified number of times for a given carrier frequency(examples of 256 and 512 were given above). In one arrangement thevalues for the carrier frequency can be selected from a look-up tableaccessible to the controller 12 (e.g. stored in local memory or on thecomputer 14), this look-up table being particular to a given antennaarray 500 a, 500 b.

Operation of the radar system 1 described above will now be describedwith reference to FIG. 12, which is a schematic flow diagram showingsteps carried out by the controller 12. At step S12.1 the controller 12energises one of the input feeds I_(k,n) of the antenna structure 701,e.g. by appropriate configuration of the switch 18; at S12.3 thecontroller 12 retrieves the value of the first carrier frequency f_(c1)(e.g. from the look-up table mentioned above), and at step S12.5 thecontroller 12 sets the values of R1 and R2 accordingly (to set thecarrier frequency) and causes the third circuit portion 230 to generatethe ramp pattern a predetermined number of times Rmp_(max) (S12.7), torepeatedly modulate the carrier frequency. Having reached Rmp_(max), thecontroller retrieves the value of the next carrier frequency f_(c2) andsets the values R1, R2. Preferably the overall duration of step S12.7—inother words the duration of any given set of repetitions of the linearramp 301 _(i) pattern—is the same for all values of the carrierfrequency, f_(cj). These steps are repeated, as shown in FIG. 12, foreach feed point I_(1,1) I_(2,1) I_(2,2) I_(2,1) to the antenna structure701, thereby causing the antenna structure 701 to progressively scanover the angular extent R.

The description has thus far focussed on the generation and transmissionof signals from the radar system 1; referring to FIGS. 1, 6, 7, 9 and10, aspects of with receiving and processing of signals will now bedescribed. As can be seen from these Figures the radar system 1preferably also includes a separate antenna 32 embodied as structure 703for receiving radiation, which corresponds to the transmitting antennastructure 701 described above. Referring to FIG. 6 or 9, the signalsreceived by antenna structure 703 are input to mixer 34, together withthe output f_(OUT) from the RF frequency generator 16, and, inaccordance with standard homodyne operation, the output from the mixer34 is fed through an ADC 36 to produce a digitised IntermediateFrequency (F_(if)) signal as input to the signal processor 38.Energising of the receiving antenna structure 703 is performed undercontrol of the controller 12, via switch 18′, and, as for thetransmitting antenna structure 703, this occurs during the linear rampperiod only 301 _(i).

The signal processor 38 is conveniently embodied as a programmable logiccontroller (PLC) and a plurality of software components, which runlocally on the PLC 38 in response to signals received from aconventional PC computer 14 and which are written using the proprietaryprogramming language associated with the PLC 38.

As described above, the radar system 1 operates according to homodyneprinciples, which means that the Intermediate Frequency F_(if) is equalto differences between the received signal frequency and the transmittedsignal frequency. In embodiments of the invention, as will beappreciated from the foregoing and FIGS. 2 and 3 in particular, theoutput of the radar system 1 is a sequence of frequency sweeps 301 _(i).It is a well known principle of radar that targets located in the pathof a given transmitted beam will reflect the transmitted signals; sincethe transmitted signal in embodiments of the present invention comprisesa linear frequency sweep 301 _(i), the reflected signals also comprise alinear frequency sweep. Targets that are stationary will generatereflected signals that are identical to the transmitted signals (albeitsomewhat attenuated), but separated therefrom at a constant frequencydifference referred to herein as a tone. Referring to FIG. 13, it willbe appreciated from the Figure that different targets T1, T2—located atdifferent distances from the radar system 1—reflect the transmittedsweep 301 _(i) at different delays in relation to the time oftransmission, and that therefore targets T1, T2 at these differentlocations will be associated with different tones Δf₁, Δf₂.

In view of the fact that the signals output from the mixer 34 containtones, the signal processor 38 is arranged to delay the processing ofsignals until the ramp 301 has travelled to the extents of the detectionregion and back. Thus for example, if the detection region extended to4.5 km from the radar system 1, the signal processor 38 would startprocessing signals output from the mixer 34 at:

$\frac{4500 \times 2}{3 \times 10^{8}} = 30$

μs from the start of transmission of a given ramp 301 _(i).

Considering, for the sake of clarity, one processing period 1301 ₁, thesignal processor 38 essentially calculates the Doppler frequency oftargets within range of the transmitted beam—and which reflect thetransmitted beam. This is achieved by sampling the received tones Δf₁,Δf₂ . . . Δf_(m) at a predetermined sampling rate. The sampling rate isselected so as to as ensure that phase shifts of the transmitted signal,which are induced by moving targets, can be captured. The skilled personwill appreciate that this is dependent on the ramp rate, since theDoppler frequency is dependent on the frequency of the transmittedsignal:

Doppler Frequency=2vf _(c) /c  [Equation 1]

Thus, the output of the ADC 36 falling within the processing period 1301₁ will be processed a predetermined number of times (corresponding tothe sampling rate) by the signal processor 38. Each sample will containzero, one or a plurality of tones, each relating to signals reflectedfrom targets.

As will be appreciated from the foregoing, the linear ramp 301 _(i) istransmitted a plurality of times for each carrier frequency. Accordinglythe signal processor 38 processes data received during a correspondingplurality of processing periods 1301 _(i), and generates, by means of aRange FFT, a set of return samples, individual members of which areassigned to a respective set of range gates for each processing period1301 _(i). Thus the output of the Range FFT, for a given Processingperiod 1301 ₁, is frequency information distributed over so-called rangegates. As is well known in the art, range gates represent successivedistances from the radar system 1, such that if return samples fallwithin a given range gate, this indicates the presence of a targetlocated at a distance equal to the range gate within which the returnsample falls.

Having transformed the received signals into range gates the signalprocessor 38 is arranged to take the FFT of the return samples assignedto each given range gate. In the current example it will be appreciatedthat each set of range gates corresponds to transmission of a linearramp 301 _(i) (for a given carrier frequency), and that the samplingrate in relation to range gates—the rate at which data falling within agiven range gate are measured—is the frequency at which the pattern oftransmission of linear ramps 301 _(i) is repeated (commonly referred toas the Pulse Repetition Frequency (PRF)). In the example given above,and with reference to FIG. 3, this is nominally 8 KHz. Accordingly, foreach carrier frequency, the signal processor 38 effectively generates anarray of data, each row in the array corresponding to a given processingperiod 1301 _(i), and each column in the array corresponding to a givenrange gate.

The FFT output comprises amplitudes and phases of various components ofsignal energy which fall on frequencies spaced linearly at the inverseof the duration of a complete signal sample set (in embodiments of theinvention, the signal set comprises tones, not absolute frequencyvalues). In the current example, therefore, and assuming the signalsample set for a given carrier frequency to comprise the 512 linearramps 301 ₁ . . . 301 ₅₁₂ transmitted at a rate of 8 KHz, there are 512FFT output bins spaced at a Doppler frequency of 8000/512=15.625 Hz; fora carrier frequency of 15 GHz, this is equivalent to 0.15625 m/s. Thuseach FFT output bin represents a different velocity; stationary targetswill appear in bin 0, while moving targets will appear in a bindependent on their velocity (a target travelling at 10 m/s will appearin bin 64).

As is known in the art, the signal processor 38 can be arranged to storeeach set of range gate samples in a “row” of a conceptuallyrectangularly-organised memory, referred to as a corner store, each rowcorresponding to range gates falling within a given processing periods130 _(i) and thus to a particular linear ramp 301 _(i). Once all 512linear ramps 301 ₁ . . . 301 ₅₁₂ have been transmitted, each column—i.e.each range gate—is read out and input to a FFT for processing thereby inthe manner described above.

From Equation 1, it will be appreciated that the Doppler frequency isdirectly proportional to the carrier frequency f_(c). Therefore when thecarrier frequency varies—as is the case with frequency scanningantennas—the variation in carrier frequency will modify the derivedDoppler frequencies so as to effectively scale the magnitude of thefrequencies. For example, a radar system that operates between 15.5 GHzto 17.5 GHz can generate Doppler frequencies, for a given target, whichvary by ±6%. This equates to a system-generated shift in Dopplerfrequency of more than 2 semitones, and a variation in ambiguous Dopplervelocity from 70 mph to 79 mph, which can complicate the task ofremoving velocity ambiguity from targets moving at these speeds andabove. Referring back to FIG. 11 it will be appreciated that in certainconfigurations of the radar system 1 the carrier frequency can jump fromthe maximum carrier frequency to the minimum carrier frequency, causingthe signal processor 38 to output a change in tone of more than 2semitones.

Accordingly the controller 12 is arranged to modify the sweep repeatperiod 307 (or sweep repetition frequency) such that the sweeprepetition frequency is proportional to the carrier frequency, therebyeffectively removing this systematic aberration. Turning back to FIG.12, this means that step S12.5 performed by the controller 12 inrelation to carrier frequency f_(cj) retrieved at step S12.3 isaccompanied by calculation of a sweep period 307 for the particularvalue of this carrier frequency f_(cj). In preferred embodiments of theinvention the linear sweep period 301 remains unchanged (so that theeffect of this adjustment does not affect the signal processor 38), andthe controller 12 adjusts the duration of the flyback and/or dwellperiods 303, 305; most preferably the dwell period 305 is modified. Ofcourse all of the repetitions of the sweep repeat period 307 ₁, f_(cj) .. . 307 ₅₁₂, f_(cj) are identical for a given carrier frequency f_(cj)(step S12.7). In one embodiment the controller 12 has access to alook-up table, which lists sweep repeat periods 307 _(j) for discretecarrier frequencies f_(cj). Conveniently such data could be stored inthe look-up table that is accessed by the controller at step S12.3, whenidentifying a next carrier frequency f_(cj).

As described above in relation to FIG. 12, the overall duration D ofstep S12.7 is preferably maintained constant. When, as is the case withembodiments of the invention, the sweep repeat period 307 _(j) varies inaccordance with carrier frequency f_(cj) the duration of 512 repetitionsapplied in respect of each different carrier frequency varies; thus, ofitself, the period associated with 512 repetitions would not be ofduration D for all carrier frequencies. In order to ensure that theduration is nevertheless constant, the controller 12 is configured towait for a period equal to the time difference between the end of 512repetitions and duration D before moving onto the next instance of stepsS12.3, S12.5 and S12.7 (i.e. for a different carrier frequency). In thepresent example the value of D is preferably set to the sum of 512 sweeprepeat periods 307 corresponding to the duration of the longest sweeprepeat period (and thus that associated with the lowest carrierfrequency f_(cj)).

This feature of the controller 12 is advantageous for configurations inwhich the linear ramp period 301 is constant (in FIG. 3 it is shown as64 μs), incurring a fixed transmitter power dissipation: maintainingduration D for the overall duration of step S12.7 means that the averagetransmitter dissipation is constant and independent of variations to thesweep repeat period 307 (PRF). As a result the temperature of thetransmitter T_(x) is maintained at a constant level, which, in turn,minimises the variations in parameters that are temperature dependent.

Preferably the Doppler frequencies are scaled and output as tones withinthe audible range and at a fixed audio sample rate. Playing back thetones at a fixed rate is a convenient approach in view of the fact thatthe Doppler frequencies have been normalised in relation to thevariation in carrier frequency.

As an alternative to selecting sweep repeat periods 307 _(j) as afunction of carrier frequency f_(cj), the sweep repeat period 307 _(j)could be varied incrementally, for example linearly, based on theapproximation 1+α≈1/(1−α) for α<<1. For the example frequency range of15.5 GHz-17.5 GHz, the sweep repeat period 307 for a carrier frequencyof 15.5 GHz could be 140.65 μs, and period 307 for a carrier frequencyof 17.5 GHz could be 125 μs, while the sweep repeat period 307 forcarrier frequencies between the extents of this range can be selected soas to vary linearly between 125 μs and 140.65 μs. As for the firstalternative—where the sweep repeat period 307 is varied discretely asthe carrier frequency varies—the linear ramp 301 and thus the processingperiods 1301 remain unchanged for all values of the sweep repeat period307. The net change in Doppler frequency is then reduced to ±0.2% andthe ambiguous Doppler velocity varies from 78.7 mph to 79.0 mph.

As described above, a radar system according to embodiments of theinvention can conveniently be used for transceiving radio frequencyenergy and processing the same so as to output an audible representationof Doppler frequencies and thus identifying moving targets. The signalprocessor 38 is arranged to transmit data indicative of the Dopplerfrequencies to the computer 14, which comprises a suite of softwarecomponents 39 arranged to convert the Doppler frequencies to audiblesignals and to playback the same. As described above, the DopplerFrequencies are normalised by processing the received signals at avariable rate, the rate being selected in dependence on the carrierfrequency of the transceived signal, while the rate at which the audiois played back is substantially constant. Preferably the post processingsoftware components 39 are arranged to ensure smooth transition betweenrespective audio bursts by controlling the playback rate in relation tothe rate at which, for a given range gate, data have been processed bythe signal processor 38 (i.e. the frequency at which the pattern oftransmission of linear ramps 301 _(i) is repeated). If the PRF is variedbetween 7 KHz and 8 KHz and the audio playback rate is 8.5 KHz, then inthe absence of suitable phased-audio control, there will be gaps in theaudio output, which presents an interruption to any audible analysis ofthe Doppler data; one way of mitigating this is to recycle Doppler dataduring periods that would otherwise be silent, until such time asfurther Doppler data are made available from the signal processor 38. Inorder to ensure a smooth transition between respective sets of Dopplerdata, the computer 14 would be arranged to fade-out previous, andfade-in and current, sets of Doppler data. As an alternative, the audioplayback rate could be set at a value lower than the PRF (e.g. for thecurrent example, 6.9 KHz) so that respective sets of Doppler dataoverlap; the periods of overlap can be managed using appropriatelyselected fade-in and fade-out functions.

In arrangements where the duration of sets of repetitions of the linearramp period 301 _(i) is constant (duration D), any set of Doppler data(corresponding to a given carrier frequency f_(cj)) will arrive at thesignal processor 38 a constant rate, which means that the softwarecomponents 39 can be configured to apply the same conditions in relationto overlaps and/or gaps in the Doppler data (since the amount of overlapor gap can always be calculated from duration D). An advantage of thisarrangement is that it simplifies the logic associated with thepost-processing software components 39 and enables more constant audiooutput over the varying PRF.

A particular feature of a radar system according to embodiments of theinvention is that the software components 39 are arranged to transmitdata output from the signal processor 38 to a remote processing system,for tracking and monitoring of targets. Most preferably the softwarecomponents 39 are arranged to transmit data output from the signalprocessor 38 each time the carrier frequency—and thus region beingscanned—changes. This means that the computer 14 acts primarily as aconduit for data, while the data intensive processes of correlatingtargets between successive scans, rendering of targets upon a displayand prediction of target behaviour can be performed by a separateprocessing system. In a preferred arrangement the data are transmittedwirelessly, but it will be appreciated that any suitable transmissionmeans could be used.

Additional Details and Alternatives

Whilst in the foregoing the linear ramp 301 is independent of variationsin the sweep repeat period, the controller 12 could alternatively modifythe duration and/or slope of the linear ramp. Whilst this is not apreferred method, because operation of the signal processor 38 (inparticular in relation to the processing periods 1301) would have to bemodified, modifying the slope is a convenient method when more than oneradar system is being utilised in a given region, since the differencein slopes of the linear ramp can be used to distinguish between outputfrom respective radar systems.

FIG. 14 shows an alternative configuration of the radar system 1comprising antenna structures according to embodiments of the invention,in which the single amplifiers 20, 20′ are replaced by individualamplifiers, each being associated with a respective antenna.

In the above passages the radar system 1 is assumed to comprise aseparate transmit and receive antenna structure 701, 703. However, andturning to FIG. 15, the radar system 1 could alternatively comprise asingle antenna structure 701 and a circulator 40, which, as is known inthe art effectively combines signals that are transmitted and receivedfrom the antenna structure 701. As an alternative to the circulator 40,the radar system 1 could include a switch or an alternative antennautilising a turnstile junction or orthomode junction (not shown).

FIG. 16 shows an alternative configuration of the antenna arrays 500 a,500 b within an antenna structure 701, in which each the antenna array500 a, 500 b is located on a respective support structure, an outer edge531 a of one support structure abutting a corresponding outer edge 531 bof another support structure so as to form an antenna structure having agenerally isosceles shape; since the supports of respective antennaarrays abut one another the radar system can be fabricated such thatreceiving antenna structure 701 abuts transmitting antenna structure703, thereby generating a physically smaller radar system, in terms ofdepth occupied by the antenna structure, compared to that shown in FIG.7. It will be appreciated that other configurations are possible,involving two, three or several such antenna arrays mounted on suitablesupport structures.

Whilst in embodiments of the invention the radar system 1 preferablyuses antenna structure 701 described above, which is based on travellingwave antenna technology, the radar system 1 could alternatively use awaveguide in the form of a serpentine antenna or similar as thefrequency scanning antenna. A suitable antenna is described in U.S. Pat.No. 4,868,574.

Whilst the above embodiments describe use of a frequency scanningantenna for beam steering, it will be appreciated that theconfigurations and methods described above could be applied for thepurposes of avoidance detection, and/or in the presence of other radarsystems, and/or to counteract frequency jamming equipment (e.g. byhopping between operating frequencies in order to avoid detection of,interference with, or jamming of, the radar system).

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

1. A frequency modulated continuous wave (FMCW) radar system comprisinga frequency generator and a homodyne receiver arranged to processsignals received from a target so as to identify a Doppler frequencyassociated with the target, wherein the frequency generator is arrangedto generate a plurality of sets of signals, the signals within a givenset having the same carrier frequency and each set having a differentcarrier frequency, the frequency generator comprising a digitalsynthesiser arranged to frequency modulate a continuous wave signal of agiven carrier frequency by a sequence of modulation patterns whereby togenerate a said set of signals, said individual ones of said patternsbeing repeated within a set, wherein the receiver is arranged to processsignals indicative of radiation reflected from a target so as to derivefrequency data therefrom, said derived frequency data representing atone corresponding to said target.
 2. A FMCW radar system according toclaim 1, wherein the receiver comprises a signal processor arranged toperform a first transform on the received signals to derive saidfrequency data representing said tone, said tone being related to arange of the target, and a second transform on data output from thefirst transform to derive a Doppler frequency related to a velocity ofthe target.
 3. A FMCW radar system according to claim 1, the receivercomprising signal processing means arranged to process said tone so asderive data indicative of movement of said target, and to transmit saidderived movement data to a display means, said display means beinglocated remote from said scanning radar system.
 4. A FMCW radar systemaccording to claim 3, wherein the radar system is arranged tocommunicate wirelessly with said display means.
 5. A FMCW radar systemaccording to claim 1, the radar system being arranged to cooperate witha portable power source.
 6. A FMCW radar system according to claim 5,wherein the portable power source comprises a battery pack or a solarpanel.
 7. A FMCW radar system according to claim 1 wherein said sequenceof modulation patterns is generated by direct digital synthesis.
 8. AFMCW radar system according to claim 1, wherein the frequency generatorcomprises a first circuit portion and a second circuit portion, thefirst circuit portion comprising a variable frequency oscillatorarranged to output signals at an output frequency in dependence oncontrol signals input thereto and tuning means arranged to generate saidcontrol signals on the basis of signals received from the second circuitportion for use in modifying operation of the variable frequencyoscillator, the second circuit portion being arranged to receive saidoutput signals and to derive therefrom signals to be input to saidtuning means, the second circuit portion comprising a frequency dividerarranged to generate signals of a divided frequency, lower than saidoutput frequency, wherein the second circuit portion comprises meansarranged to derive reduced frequency signals from said output signal,said reduced frequency signals being of a frequency which is lower thansaid output frequency and higher than said divided frequency.
 9. A FMCWradar system according to claim 8, further comprising a fixed frequencyoscillator, wherein the second circuit portion comprises a staticfrequency multiplier component arranged to derive increased frequencysignals from the fixed frequency oscillator.
 10. A FMCW radar systemaccording to claim 9, wherein the means arranged to derive reducedfrequency signals comprises a mixer component arranged to receivesignals output from the variable frequency oscillator and signals outputfrom the static frequency multiplier component so as to derive saidreduced frequency signals.
 11. A FMCW radar system according to claim 9,wherein the tuning means comprises a frequency comparator component,said fixed frequency oscillator being arranged to output signals to thefirst circuit portion via a further frequency divider, and saidfrequency comparator being arranged to generate said control signals onthe basis of signals received from said further frequency divider andsaid divided frequency.
 12. A FMCW radar system according to claim 8,wherein the frequency generator further comprises: combining meansarranged to combine said sequence of modulation patterns with saidsignals output from the first circuit portion so as to frequencymodulate said signals output from the first circuit portion.
 13. A FMCWradar system according to claim 1, further comprising a radar controllerfor use in controlling modulation of the continuous wave signal, whereinthe radar controller is in operative association with the digitalsynthesizer so as to modify a given sequence of modulation patterns independence on the carrier frequency of the signal being modulated.
 14. AFMCW radar system according to claim 13, wherein the radar controller isarranged to modify the duration of individual modulation patterns of thesequence, thereby modifying the modulation pattern.
 15. A FMCW radarsystem according to claim 13, wherein each modulation pattern of thesequence comprises a ramp period and an intervening period, and theradar controller is arranged to modify the duration of the interveningperiods of respective modulation patterns in the sequence, therebymodifying the modulation pattern.
 16. A FMCW radar system according toclaim 15, wherein each modulation pattern of the sequence comprises alinear ramp period and a dwell period, and the radar controller isarranged to modify the duration of dwell periods of respectivemodulation patterns in the sequence, thereby modifying the modulationpattern.
 17. A FMCW radar system according to claim 15, wherein eachmodulation pattern of the sequence comprises a linear ramp period and adescent period, and the radar controller is arranged to modify theduration of descent periods of respective modulation patterns in thesequence, thereby modifying the modulation pattern.
 18. A FMCW radarsystem according to claim 13, wherein each modulation pattern of thesequence comprises a linear ramp period.